The present invention relates to a facsimile communication system, and more particularly to a facsimile communication system suitable for a circumstance where different types of facsimile machines are used in a store and forward switching network such as a packet switching network.
In a conventional facsimile communication system which uses a store and forward switching network such as a packet switching network, a G3 facsimile machine cannot be directly connected to the packet switching network. As is know, packet switching is a data transmission process utilizing addressed packets whereby a channel is occupied only for the duration of transmission of the packet. A packet is a group of binary digits including data and control elements which is switched and transmitted as a composite whole. A G3 facsimile machine cannot receive data directly from a packet switching network. In order for a G3 facsimile machine to make use of the data in a packet the packet must be disassembled to its original form. Also, in order for a G3 facsimile machine to transmit data over a packet switching network, the data must be assembled into packets with the appropriate control and data elements. Such packet assembly and disassembly functions are performed by a device known in the art as a Packet Assemble and Disassembly (PAD) device. As shown in an article "Separate System for Facsimile Communication", STUDY OF INTERNATIONAL COMMUNICATION, No. 128, April 1986, pp 73-79, published by Kokusai Denshin Denwa Co. Ltd., a G3 facsimile packet assembly and disassembly (PAD) device is provided between the G3 facsimile machine and the packet switching network (G3 facsimile communication system). A standard specification of a G4 facsimile machine was recommended in 1984 by CCITT as T.5 and T.6. This G4 facsimile machine can be directly connected to the store and forward switching network such as packet switching network.
The G3 facsimile machine and the G4 facsimile machine have different design philosophies and there are many differences between them as shown in FIG. 1. In order for the G3 facsimile machine and the G4 facsimile machine to communicate with each other through the store and forward switching network, it is necessary to absorb in the system all differences shown in FIG. 1 except for the facsimile signal form in item 3. The conversions of communication control system, terminal control system, modulation technique, transmission rate, transmission error control, frame configuration and one-line transmission time of the items 1, 2, 4, 5, 7, 8 and 11 can be attained in the facsimile PAD device, but the conversions of redundancy compression, resolution and line synchronization signal of the items 6, 9 and 10 cannot be attained in the PAD device. Thus, the prior art network pays no attention to the circumstance where the G3 facsimile machines and the G4 facsimile machines are mixedly connected to the store and forward switching network, and even if a G4 facsimile machine is connected to the store and forward switcher, the G3 facsimile machine and the G4 facsimile machine cannot communicate with each other.
In order to solve those problems, it has been proposed to absorb the differences shown in FIG. 1 in the store and forward switching network, but the store and forward switcher must communicate with a different protocol depending on whether the machine is a G3 facsimile machine or a G4 facsimile machine, and hence the control is complex. Since the facsimile machine handles a large volume of data, a load is heavy in encoding algorithm conversion or resolution conversion, and a throughput of the system is lowered because of a limitation to a processing performance of the store and forward switcher. Such a problem usually occurs in mutual communication between different types of facsimile machines.